In conventional diagnostic ultrasound systems, as described in Japanese Patent No. 3194947 and U.S. Pat. No. 5,143,070, when an examining physician such as a paramedic applies extraneous force to a body surface of a region-of-interest in a subject using a probe, a difference between ultrasonic cross-section images of different frames is produced in order to measure a displacement caused by applying the extraneous force. The conventional arts make it possible to diagnosis hardness or softness of each biological tissue in the region-of-interest.
However, the above patent documents merely describe displaying a contour image of distortion and Young's modulus values. The conventional diagnostic ultrasound systems cannot cope with the need for time-sequentially acquiring ultrasonic images while varying the pressure applied to a body surface by giving force to a probe, and producing a difference between ultrasonic images so as to visualize the hardness or softness of a biological tissue.